Thomas Stroman, Martin Pohl, Jacek Niemiec, Antoine Bret
There is an observational correlation between astrophysical shocks and
non-thermal particle distributions extending to high energies. As a first step
toward investigating the possible feedback of these particles on the shock at
the microscopic level, we perform particle-in-cell (PIC) simulations of a
simplified environment consisting of uniform, interpenetrating plasmas, both
with and without an additional population of cosmic rays. We vary the relative
density of the counterstreaming plasmas, the strength of a homogeneous parallel
magnetic field, and the energy density in cosmic rays. We compare the early
development of the unstable spectrum for selected configurations without cosmic
rays to the growth rates predicted from linear theory, for assurance that the
system is well represented by the PIC technique. Within the parameter space
explored, we do not detect an unambiguous signature of any cosmic-ray-induced
effects on the microscopic instabilities that govern the formation of a shock.
We demonstrate that an overly coarse distribution of energetic particles can
artificially alter the statistical noise that produces the perturbative seeds
of instabilities, and that such effects can be mitigated by increasing the
density of computational particles.
View original:
http://arxiv.org/abs/1201.5932
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